1. Field of the Invention
This invention relates generally to a system and method for estimating a state-of-charge (SOC) of a battery, and, more particularly, to a system and method for estimating the SOC of a battery using changes in the size or pressure of the battery.
2. Discussion of the Related Art
Electric vehicles are becoming more and more prevalent. These vehicles include hybrid vehicles, such as extended range electric vehicles that combine a battery and a main power source, such as an internal combustion engine, fuel cell system, etc., and electric only vehicles, such as battery electric vehicles. All of these types of electric vehicles employ a high voltage battery that can be different battery types, such as lithium-ion, nickel metal hydride, lead acid, etc. The battery system can include individual battery modules where each battery module may include a certain number of battery cells, such as twelve cells.
Because batteries play an important role in powering electric vehicles and hybrid vehicles, effective battery control and power management is essential to vehicle performance, fuel economy, battery life and passenger comfort. Accurate knowledge of the SOC is critical for proper control of the battery system in an electric vehicle to obtain long battery life and good fuel economy. Because the SOC cannot be directly measured while operating the vehicle, a battery controller needs to predict and estimate the SOC in real-time using other battery parameters such as open circuit voltage and current.
It is well known by those skilled in the art that battery dynamics are generally non-linear and highly dependent on battery operating conditions, which means that an accurate estimation of battery SOC cannot be guaranteed. One approach to estimate the SOC of a battery is to monitor the battery's open circuit voltage. In general, the higher the open circuit voltage the higher the SOC. However, open circuit voltage is inherently difficult to use to accurately estimate the SOC because the battery voltage is influenced by many factors, not just SOC, for example, the temperature, short term charging history, long-term vehicle driving history, age of the battery, etc. For most battery cell chemistries, the voltage level decreases only slightly, if at all, as the battery starts discharging. At some point at a lower SOC the voltage level starts to decrease at a faster rate.
Lithium-ion batteries have proven to be promising for hybrid electric vehicles. Estimating the SOC is significantly more challenging for lithium-ion batteries than the older nickel based technology because lithium-ion based batteries maintain their voltage level for a long time even as the SOC drops. The voltage of a lithium ion battery will not change significantly in a range from about 20% to 80% SOC.
Another way the battery controller can estimate SOC is to calculate the electric charge flowing into and out of the battery by integrating the current over time. One problem with this approach is that the estimated SOC drifts away from the real SOC over time. Therefore, the battery controller needs to reset or readjust the estimated SOC periodically to match the real SOC. One way to reset the estimated SOC is to charge the battery to 100%. However, the vehicle driver may charge the battery when the SOC is down to 30%. The driver may charge the battery for the next trip, but at the start of the next trip, the battery might have been recharged only to 70% SOC. The vehicle could then be driven until the battery drains to 40% SOC and then charged again, still not reaching 100% SOC before the vehicle is off on another trip. Given this type of scenario, resetting the SOC when the charge is at 100% is problematic. Another option is to discharge the battery to 0% SOC, but, as with charging to 100% SOC, this is detrimental to the battery.
What is needed is a way to estimate the SOC of a battery that overcomes the limitations of the current SOC estimation techniques.